Safety system and method for cutting machine

ABSTRACT

A safety system prevents cutting machine operators from being accidentally carried through the infeed chute into the machine&#39;s cutting mechanism. An operator wears a safety device on his wrist and/or ankle and a sensor array mounts on opposing sides of the cutting machine&#39;s infeed chute. The sensor array elements are wound in opposite directions and connect in series opposing. The magnetic field of the safety device induces a current in the sensor array as it moves in the proximity of the sensor array. The safety system generates a signal which is proportional to the sum of the induced currents. When the signal exceeds a threshold, the safety system shuts off power to the feed mechanism and/or the cutting blades of the cutting machine, preventing injury to the operator.

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Patent Application No. 61/424,570, filed on Dec. 17,2010 and entitled “Safety Device for Cutting Machine,” the entirety ofwhich is hereby incorporated herein by reference to be considered partof this specification.

BACKGROUND

This invention relates, generally, to automatically stopping theoperation of machinery, and more particularly, to cutting machines, suchas wood chippers having one or more feed wheels for controlling theinfeed of bulk wood products, one or more cutting blades which createand direct the produced wood chips toward a discharge chute, and anemergency safety device with a sensor array located in the infeed chutethat directs control circuitry to stop the motive power directed to thefeed wheels, the cutting blade, or both

Cutting machines such as wood chippers are used to reduce branches,trees, and other bulk wood products into small wood chips. A typicalwood chipper often contains an infeed chute, a feed system forcontrolling the feed rate of wood products, a wood chipping mechanism, adrive system for the feed system and chipping mechanism, and a dischargechute. The infeed chute is typically a funnel-type conduit provided witha wide opening which tapers toward the feed system to converge the bulkwood products toward the chipping mechanism.

Through the action of the feed system, the bulk wood products arebrought into contact with the chipping mechanism which grinds, flails,or cuts the wood products into small pieces and propels the small piecesinto the discharge chute where they exit the wood chipper.

These types of wood chippers are, if operated incorrectly, dangerousdevices. The chipping mechanism typically rotates at a high speed andproduces high torques, which are necessary to chip the wood. The feedsystem located at the narrowest point of the infeed chute is a dangerousarea which can catch a user's clothing or, more importantly, a user'slimb if he improperly reaches into the infeed chute during operation ofthe chipper. If a user does get entangled in the feed system of knownchippers, the user may not be able to reach a shutoff actuator locatedoutside of the chute. Indeed, serious injuries continue to occur tooperators of these devices.

SUMMARY

A wood chipper according to an embodiment of the invention incorporatesa safety device to cut off power to a feed system, a cutting mechanism,or both. Certain embodiments can be used with a wide variety of wastereducing machinery which receives waste products through an infeedchute. Other embodiments can be used with a wide variety of machinery toautomatically disable their operation.

According to one embodiment, a sensor array having first and secondsensor elements is mounted within the infeed chute of the machinery.Each element of the sensor array detects the presence of a magneticfield which generates a current in each sensor element. For magneticfields generated outside the sensor array, such as the magnetic field ofthe earth, the sensor array subtracts the currents generated in thefirst and second elements, thus canceling any effects thereof. Formagnetic field generated inside the sensor array, such as a safetydevice comprising a magnet moving between the sensor array elements, thesensor array adds the currents generated in the first and secondelements. This current or its corresponding voltage potential isamplified and compared to a threshold. When the current/voltagepotential exceeds the threshold, control circuitry generates a shut-offsignal, which disables operation of the machinery.

Furthermore, users of the machinery wear safety devices, such as gloves,wrist bands and/or ankle bracelets having magnets mounted therein. Themagnets mounted in the safety devices produce the magnetic fielddetected by the sensor array. The sensor array detects the motion of themagnetic field between the sensor array elements, which are mounted onthe infeed chute, and automatically notifies the control circuitry tostop the operation of the machine. For example, if a person wearing theglove type safety device moves his hand within the infeed chute of awood chipping machine having sensor array elements mounted thereon, thesensor array will detect that the magnet mounted in the user's gloves iswithin the infeed chute. In response, the sensor array notifies thecontrol circuitry to stop the operation of the feed system and/or thecutting mechanism.

Certain embodiments relate to a method of automatically interrupting theoperation of a cutting machine including forming a sensor arrayincluding a first sensor element and a second sensor element. The firstsensor element including a first winding wound in a first direction. Thesecond sensor element including a second winding wound in a seconddirection. The first and second windings connected in series opposing.The method further comprising mounting the sensor array on opposingsides of an infeed chute of a cutting machine such that the first sensorelement is mounted to a first side of the infeed chute and the secondsensor element is mounted to an opposing second side of the infeedchute, and sensing a current induced in the windings by a magneticfield. The magnetic field produced by a safety device placed proximateto the sensor array. The method further comprises comparing a signalproportional to the induced current to a threshold, and terminating anoperation of the cutting machine when the signal exceeds the threshold.The method further comprising inducing a first current in the firstwinding from the safety device positioned between the windings, inducinga second current in the second winding from the safety device positionedbetween the windings, adding the first and second induced currents,inducing a third current in the first winding from an external source,inducing a fourth current in the second winding from the externalsource, and subtracting the third current and the fourth current.

According to a number of embodiments, the disclosure relates to a safetysystem for a cutting machine. The safety system includes a sensor arrayincluding at least two sensor elements. Each sensor element comprises aprotective plate and a winding. The winding comprises at least one loopof wire. The sensor elements mount on opposing sides of an infeed chuteof a cutting machine and connect in series opposing. The safety systemfurther comprises a signal processor, which comprises an amplifier toamplify a signal proportional to a current induced in the windings. Thecurrent induced by a magnetic field moving proximate to the sensorarray. The signal processor further comprises a comparator to comparethe amplified signal to a threshold value such that the signal processorcauses the cutting machine to shut off power to an operation of thecutting machine when the amplified signal is greater than the thresholdvalue.

In accordance with various embodiments, an apparatus for automaticallyinterrupting operation of a cutting machine comprises a sensor arrayincluding at least a first array element and at least a second arrayelement. Each array element includes a winding. The array elementsconnect in series opposing. The apparatus further comprises an amplifierfor amplifying a signal proportional to a sum of a current induced by amagnetic field in the at least first sensor array element and a currentinduced by the magnetic field in the at least second sensor arrayelement, a threshold circuit for generating a predetermined threshold, acomparator for comparing the amplified signal to the predeterminedthreshold, and a shut-off circuit to terminate an operation of a machinewhen the amplified signal is greater than the predetermined threshold.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the embodiments have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment of the invention.Thus, the inventions may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the drawings, reference numbers are re-used to indicatecorrespondence between referenced elements. The drawings, associateddescriptions, and specific implementation are provided to illustrateembodiments and not to limit the scope of the disclosure.

FIG. 1 illustrates a cutting machine safety system, according to certainembodiments.

FIG. 2 illustrates an exemplary sensor array, according to certainembodiments.

FIG. 3 illustrates a block diagram of an exemplary safety system todisable an operation of a cutting machine, according to certainembodiments.

FIG. 4 illustrates a flowchart of an exemplary process for terminatingan operation of a cutting machine, according to certain embodiments.

FIGS. 5A-5C illustrate an exemplary circuit for a cutting machine safetysystem, according to an embodiment.

FIG. 6 illustrates an exemplary glove-type safety device, according tocertain embodiments.

FIG. 7 illustrates an exemplary safety device worn on the user's limb,according to an embodiment.

FIG. 8 illustrates exemplary ankle bracelet safety devices worn aroundthe user's shoes, according to an embodiment.

DETAILED DESCRIPTION

The features of the inventive systems and methods will now be describedwith reference to the drawings summarized above. Although certainembodiments are described with respect to safely operating a woodchipper, other embodiments can be used to stop the operation ofmachinery where an operator or operator's limb is within an unsafedistance from mechanisms that stamp, cut, roll, weld, shred, drill,bend, apply pressure, or the like. Yet other embodiments can be used tostop the operation of machinery when any foreign body is within anunsafe distance from the mechanisms.

FIG. 1 illustrates an embodiment of a cutting machine safety system 100.The system 100 comprises a cutting machine 102, such as a wood chipperor the like. The cutting machine 102 comprises an infeed chute 104, afeed mechanism 106 and cutting blades (not shown). An operator 108typically feeds material to be shredded into the infeed chute 104, whereit is captured by the feed mechanism 106. The feed mechanism 106 propelsthe material into the cutting blades located behind the feed mechanism106, where the cutting blades macerate the materials. The feed rate of awood chipper, for example, can be around 2 feet per second.

The system 100 further comprises a sensor array 110, control circuitry112, and at least one safety device 114. In the illustrated embodiment,the sensor array 110 comprises a first sensor element 110 a and a secondsensor element 110 b. In other embodiments, the sensor array 110 mayhave more than two sensor elements. In an embodiment, the sensor array110 mounts onto the inside surface of the infeed chute 104, such that atleast two sensor elements 110 a, 110 b oppose each other. In otherembodiments, the sensor array elements mount onto any surface such thatthe sensor array detects the presence of the safety device 114 passingbetween at least two of the sensor array elements 110 a, 110 b.

The control circuitry 112 processes the signals from the sensor array110 to determine whether the safety device 114, worn by the operator108, has come between the sensor elements 110 a, 110 b. Once the controlcircuitry 112 detects the safety device 114 between the sensor arrayelements 110 a, 110 b, the control circuitry 112 generates a controlsignal to stop an operation of the machine 102, such as, for example,stopping the feed mechanism 106 from advancing, the cutting blades fromcutting, or both.

The operator 108 typically wears the safety device 114 on a limb. FIG. 1illustrates safety devices 114 worn around the each of the operator'swrists and safety devices 114 worn around each of the operator's lowerlegs. The safety device 114 can also be attached to the operator'sgloves or boots.

In other embodiments, the safety device 114 can be located on anyforeign body which could cause damage to the machinery if it wasintroduced into the working parts of the machine 102. For example, ametal pole used to push material into a cutting or grinding mechanismcould damage the mechanism if it is inserted too far. By locating thesafety device 114 on the working end of the pole, the control circuitry112 would detect when the pole comes within a predetermined distance ofthe sensor array 110 and stop all or part of the machine operation.

In an embodiment, the safety device 114 comprises at least one magnet,which produces a magnet field. In another embodiment, the safety devicecomprises at least two magnets. The sensor elements 110 a and 110 bdetect the magnetic field from the magnet of the safety device 114.

FIG. 2 illustrates an exemplary sensor array 110 comprising the firstsensor element 110 a and the second sensor element 110 b. Each sensorelement or antenna 110 a, 110 b comprises a plate 202 and at least onewinding or loop of wire 204. In other embodiments, the sensor arrayelements 110 a, 110 b comprise windings or loops 204 without the plate202.

In an embodiment, the plate 202 is made of metal to withstand theextreme forces of pounding lumber and tree limbs as these materials arebeing loaded and pulled through the wood chipper 102. In an embodiment,the front side of the metal plate 202 comprises a beveled edge to reducebinding between the plate 202 and the material being fed into thechipper 102. For example, the bevel can be between approximately 20° andapproximately 60°, and preferably around 40°. The backside of the plate,which is illustrated in FIG. 2, has a routed channel or groove 206proximate to the edges, where the winding 204 lays. In embodiments wherethe machine 102 produces vibrations, the channels 206 are filled with apotting material, such as silicon glue, for example, to prevent thewinding 204 from vibrating within the channel 206. In one embodiment,the plate 202 mounts to a sidewall of the wood chipper infeed chute 106such that the winding 204 is captive between the plate face and theinfeed chute sidewall to protect the winding 204 from the materialsbeing loaded into the chipper 102.

In one embodiment, the plate 202 comprises an aluminum plate. As anon-ferrous material, aluminum advantageously does not attenuate themagnetic fields from the safety devices 114. Such attenuation wouldresult in a loss of sensitivity in the detection of the safety devices114. In other embodiments, the plate comprises other metals, such assteel, stainless steel, brass, copper, and the like, and non conductivematerials, such as plastic, methyl methacrylate,polyoxybenzylmethylenglycolanhydride, plywood, wood, and the like.

In one embodiment, the metal plate 202 has a height of approximately24½″, a width of approximately 12½″ and a thickness of approximately ½″.In other embodiments, the height can be more or less than 24½″, thewidth can be more or less than 12½″ and the thickness can be more orless than ½″. The metal plate can be square, rectangular, circular,oval, irregularly shaped, or the like.

The sensor array elements 110 a, 110 b further comprise the winding orantenna 204 wound in the groove 206 in the metal plate 202. The winding204 comprises standard insulated conductive wire which can withstand theenvironmental conditions. The wire can be single or multiple conductorwire. The gauge of the wire can range from approximately 36 AWG toapproximately 12 AWG. In another embodiment, the wire gauge number canbe smaller than 12 AWG or larger than 36 AWG. In one embodiment, 18 AWGcopper, type CL2 E60233-8, 7 conductor wire was used.

In an embodiment, each sensor array element 110 a, 110 b is wound withbetween approximately 3 turns and approximately 100 turns. In oneembodiment, each sensor array element 110 a, 110 b is wound withapproximately 14 turns. In other embodiments, the number of turns isgreater than 100 or less than 3. Each wire used in the winding 204 has astart end and a finish end.

As illustrated in FIG. 2, each sensor array element 110 a, 110 b iswound in an opposite direction, such that when the sensor array elements110 a, 110 b mount on opposing sides of the infeed chute 104, they arewound in the same direction with respect to each other. For example, thewinding 204 of sensor array element 110 a is wound clockwise and thewinding 204 of sensor array element 110 b is wound counterclockwise. Inanother example, the winding 204 of sensor array element 110 a is woundcounterclockwise and the winding 204 of sensor array element 110 b iswound clockwise.

Further, as illustrated in FIG. 2, the sensor array elements 110 a 110 bare connected in series opposing, such that a start end of one of thewindings 204 connects to a start end of the other of the windings 204 ora finish end of one of the windings 204 connects to a finish end of theother of the winding 204. For example, the start ends of each winding204 connect together and the finish ends input to the control circuitry112 as the ANTENNA IN signals. In another example, the finish ends ofeach winding 204 connect together and the start ends of each windinginput to the control circuitry 112 as the ANTENNA IN signals. In oneembodiment, the sensor array elements 110 a, 110 b connect in seriesopposing with the same number of turns, but wound in oppositedirections, such that one sensor element 110 a, 110 b is wound counterclockwise and another sensor element 110 a, 110 b is woundcounterclockwise and when the sensor elements 110 a, 110 b are mountedon opposing sides of the infeed chute 104, they are wound in the samedirection with respect to each other.

A current is induced in a loop of conductive wire when a magnetic fieldis moved toward the loop. The amount of induced current is proportionalto the number of loops in the winding 204. The amount of induced currentis also proportional to the distance between the magnetic field and theloop. Here, as the magnetic field generated by the magnet of the safetydevice 114 moves closer to and between the elements 110 a, 110 b of thesensor array 110, a current/voltage potential is induced in each winding204 of the sensor array 110. As the magnet moves closer to the sensorarray 110, the induced current increases.

Induced currents/voltage potentials from external fields of magneticflux are canceled by the sensor array 110. The Earth acts as a pointsource of magnetic radiation with respect to the array elements 110 a,110 b. With respect to the sensor array 110, the Earth radiatesequidistant magnetic flux lines through the center of each wind/loop204. The Earth's magnetic flux induces a current in the windings/loops204 when the sensor array elements 110 a, 110 b move. Since the chipper102 can vibrate substantially during operation, a current/voltagepotential will be induced in the array elements 110 a, 110 b due to theEarth's magnetic field. Because this is an external field to the safetysystem 100, the current induced in each winding 204 is of approximatelyequal strength and flows in the same direction in each winding 204.Since the windings 204 electrically connect in series opposing manner,the signals generated in each winding due to the Earth cancel. As such,the array elements 110 a, 110 b perform noise canceling for pointsources of magnetic flux.

Other sources of magnetic flux, such as an engine firing, and the like,which are far from the array 110 when compared to distance between thesafety device 114 and the array 110, will also act as point sources andthe effect of any radiated magnet flux is canceled in the sensorelements 110 a, 110 b.

Induced currents/voltage potentials from internal fields of magneticflux, such as those created by the safety device 114, are added togetherby the sensor array 110. As the safety device 114 passes between thearray elements 110 a, 110 b, the same motion of the magnetic field atthe same time to both array elements 110 a, 110 b inducescurrent/voltage potential of approximately equal and opposite strengthin each sensor array element 110 a, 110 b. Because of the seriesopposing connection of the sensor elements 110 a, 110 b, the effect ofthe moving magnetic field is additive. Thus, instead of the inducedcurrents canceling as is the case with the Earth's magnetic field, theinduced currents from the safety device 114 add together. In otherwords, the induced current/voltage potential due to the safety device114 is the sum of the magnitude of the induced current/voltage potentialin each array element 110 a, 110 b.

The control circuitry 112 amplifies and compares the differential signalto a threshold. In an embodiment, the threshold is predetermined. Inanother embodiment, the threshold is user settable. When the inducedcurrent/voltage potential exceeds the threshold, the control circuitry112 generates control signals to stop the operation of the feedmechanism 106, the cutting blades, both, or any other machine functionor mechanism.

FIG. 3 illustrates an exemplary block diagram of the safety system 100for a cutting machine 102. The sensor array 110 detects the movement ofthe safety device 114 between the array elements 110 a, 110 b and sendsa sensor signal to the signal processor 112. The sensor signal is based,at least in part, on the current induced in the sensor elements 110 a,110 b, due to the movement of the magnetic field from the safety device114 between the sensor array elements 110 a, 110 b. The signal isindirectly proportional to the distance between the safety device 114and the sensor array 110. As the distance between the safety device 114and the sensor array 110 decreases, the signal due to the inducedcurrent increases. As the distance between the safety device 114 and thesensor array 110 increases, the signal due to the magnetic fielddecreases proportional to the square of the distance.

The signal is also directly proportional to the number of loops in thewinding 204 of the sensor array elements 110 a, 110 b. As the number ofloops increase, the signal due to the magnetic field increases and asthe number of loops decreases, the signal due to the magnetic fielddecreases.

The signal processor 112 comprises a differential amplifier 302, athreshold circuit 304, a comparator 306, and a shut-off circuit 308. Thedifferential amplifier 302 receives the sensor signals ANTENNA IN(1),ANTENNA IN(2) from the sensor array 110. The sensor signals representsthe magnitude of the signal in the series opposed sensor array elements110 a, 110 b. The differential amplifier 302 amplifies the sensorsignal.

The threshold circuit 304 outputs two equal and oppositethresholdlevels. Depending on how the windings 204 of the sensorelements 110(a), 110(b) connect to form the series opposing connectionand or the makeup of the safety devices 114 with magnetic poles facing adirection and or the direction of motion of these safety devices 114within the sensor elements 110(a), 110(b) the amplified signal from thedifferential amplifier 302 may be either a positive or a negative value.The threshold levels are set to cause the safety system 100 to shut offthe cutting machine 102 when the safety device 114 is between the arrayelements 110(a), 110(b). In another embodiment, the threshold levels areset to cause the safety system 100 to shut off the cutting machine 102when the safety device 114 is within a predetermined distance of thesensor array 110. Factors that affect the signal reaching and orcrossing the threshold level include the strength, size, and number ofmagnets associated with the safety device 114, the size and number ofloops in the winding 204, the number of sensor elements 110 a, 110 b,and the like. In one embodiment, the threshold circuit 304 outputs apredefined threshold. In another embodiment, the threshold is userdefined.

The comparator circuit 306 compares the threshold set points from thethreshold circuit 304 to the amplified sensor signal from thedifferential amplifier 302. When the amplified sensor signal is greaterthan or equal to the positive threshold set point, or when the amplifiedsensor signal is less than or equal to the negative threshold set point,the comparator circuit 306 outputs a shut-off signal to the shut-offcircuit 308. In another embodiment, when the amplified sensor signal isgreater than the positive threshold set point, or the amplified sensorsignal is less than the negative threshold set point, the comparatorcircuit 306 outputs a shut-off signal to the shut-off circuit 308. In afurther embodiment, when the amplified difference signal is greater thanthe positive threshold set point or less than the negative threshold setpoint by a predetermined amount, the comparator circuit 306 outputs ashut-off signal to the shut-off circuit 308.

The shut-off circuit 308 receives the shut-off signal and generates acontrol signal to shut-off the cutting machine 102. The cutting machine102 receives the control signal and stops the feed mechanism 106, thecutting blades, or both.

FIG. 4 illustrates a flowchart of an exemplary process 400 for thesafety system 100. At block 402, the process 400 receives the sensorsignal from the sensor array 110. The received sensor signal representsthe sum of the current/voltage potential induced in each of the sensorarray elements 110 a, 110 b. At block 404, the process 400 amplifies thereceived sensor signal.

At block 406, the amplified signal is compared to the two threshold setpoints. When the amplified signal is greater than the positivethreshold, or less than the negative threshold, the process 400, atblock 408, generates a shut-off signal to interrupt operation of themachine 102. In an embodiment, when the amplified signal is greater thanthe positive threshold or less than the negative threshold by apredetermined amount, the process 400 generates the shut-off signal tointerrupt the operation of the machine 102. When the amplified signal isnot greater than the positive threshold, greater than the positivethreshold by a predetermined amount, less than the negative threshold,or less than the negative threshold by a predetermined amount, theprocess 400 returns to block 402.

FIGS. 5A-5C illustrate an embodiment of the safety system 100 comprisingthe sensor array 110 and the signal processor 112. Referring to FIG. 5A,a sensor array 502 comprises a first sensor element 502(1) and a secondsensor element 502(2). Preferably, the sensor elements 502(1), 502(2)mount on opposing sides of the infeed chute 104 of the machine 102.

The first sensor element 502(1) is wound with 14 turns of wire in aclockwise direction to form a first antenna or winding. The winding ofthe first sensor element 502(1) has a first end, Start1, and a secondend, Finish1. The second sensor element 502(2) is wound with 14 turns ofwire in a counterclockwise direction to form a second antenna orwinding. The winding of the second sensor element 502(2) has a firstend, Start 2 and a second end, Finish2. The sensor elements 502(1),502(2) connect in series opposing such that the first end of the firstwinding (Start1) connects to the first end of the second winding(Start2).

The second end of the first winding (Finish1), carrying the signalANTENNA IN(1), connects to a first end of resistor R1 504, throughterminal 1 of terminal block 508 and pin 1 of plug 6 on a printedcircuit board (PCB) 510. The second end of the second winding (Finish2),carrying the signal ANTENNA IN(2) connects to a first end of resistor R2512, through terminal 2 of terminal block 508 and pin 2 of plug 6 on thePCB 510.

A +12 volt battery 634 provides the voltage source. A positive terminalof the battery 634 couples to an anode of protection diode D2 590through a POWER on/off switch 636, terminal 4 of terminal block 508, andpin 1 of plug 5 on PCB 510. A cathode of the protection diode D2 couplesto a first end of a resistor R14 594 through a fuse F1 592. A second endof resistor R14 594 connects to a second end of R35 630.

A negative terminal of the battery 634 couples to a cathode ofprotection diode D3 578 through terminal 3 of terminal block 508 and pin2 of plug 5 on PCB 510. An anode of the protection diode D3 578 connectsto a first end of a resistor R15 580 and a second end of the resistorR15 580 connects to a second pin of C3 516 and a first pin of C4 518.

In an embodiment, battery 634 is a 12 volt battery. In otherembodiments, other DC batteries, such as for example, 9 volts to 24volts, and the like can be used. In yet other embodiments, an AC powersupply with a rectifying circuit can be used to supply power to thesignal processor 112.

In an embodiment, the POWER on/off switch 636 is a single pole singlethrow (SPST) normally open switch, or the like. In other embodiments,other switch configurations can be used as is known to one of ordinaryskill in the art. In an embodiment, when the machine 102 is powered, thesafety system 100 is also powered.

Referring to FIGS. 3 and 5B, the differential amplifier 302 comprisesoperational amplifiers U2-A 506, U2-B 514, and U2-C 540. Operationalamplifier U2-A 506 pin 4 and operational amplifier U2-D 540 pin 11connect to the positive power supply voltage and the negative powersupply voltage, respectively, to provide power to operational amplifiersU2-A 506, U2-B 514, and U2-C 540, as is known to one of skill in theart. In the embodiment illustrated in FIG. 5B, operational amplifiersU2-A 506, U2-B 514, U2-C 540, along with circuit elements R1 504 throughR11 552, and C3 516-C6 534, C23 632 and C24 616 amplify the antennainput signals ANTENNA IN(1) and ANTENNA IN(2) with a gain ofapproximately 6,400 and a roll off of approximately 5 Hz. Operationalamplifier U2-D 554 along with a capacitor C1 556 and R13 550 comprise anauto-zeroing circuit to compensate for temperature drift.

Referring to FIG. 5B, a second end of resistor R2 512 connects to apositive input of the amplifier U2-B 514. The second end of resistor R1504 connects to a positive input of the amplifier U2-A 506.

The first end of resistor R1 504 further connects to a first end ofresistor R35 630 and a first pin of capacitor C3 516. A second pin ofcapacitor C3 516 connects to a first pin of capacitor C4 518 and thesecond end of resistor R15 580. A second end of resistor R35 630connects to the second end of resistor R14 594.

A second pin of capacitor C4 518 connects to the first end of resistorR2 512, a first end of resistor R34 629, and a first end of resistor R12520. A second end of resistor R34 629 connects to the second end of R35630.

Resistors R34 629 and R35 630 compensate for the offset bias of theoperational amplifiers U2-A 506 and U2-B 514, if needed, and areselectable.

A negative input of the amplifier U2-A 506 connects to a first end ofresistor R3 522 and a second end of resistor R3 522 connects to a firstpin of capacitor C5 524, a first end of resistor R7 526, and a first endof resistor R5 528. An output of the amplifier U2-A 506 connects to asecond pin of capacitor C5 524, a second end of resistor R7 526, and afirst end of resistor R8 530. In the illustrated embodiment, capacitorC5 524 and resistor R7 526 comprise approximately a 5 Hz filter.

A negative input of the amplifier U2-B 514 connects to a first end ofresistor R4 532 and a second end of resistor R4 532 connects to a firstpin of capacitor C6 534, a first end of resistor R6 536, and a secondend of resistor R5 528. An output of the amplifier U2-B 514 connects toa second pin of capacitor C6 534, a second end of resistor R6 536, and afirst end of resistor R9 538.

A negative input of the amplifier U2-C 540 connects to a second end ofresistor R8 530, a first end of resistor R10 542, and a first pin ofcapacitor C24 616. An output of the amplifier U2-C 540 forms the signalCOMP IN and connects to a second end of resistor R10 542, a second pinof capacitor C24 616, a first end of R30 548, and a first end ofresistor R13 550.

A positive input of the amplifier U2-C 540 connects to a second end ofresistor R9 538, a first end of resistor R11 552 and a first pin ofcapacitor C23 632. A second pin of capacitor C23 632 connects to asecond end of resistor R11 552 and a first end of R31 562.

The response of amplifier U2-C 540 is based at least in part on thevalues of capacitors C23 632 and C24 616. In an embodiment, the valuesof capacitors C23 632 and C24 616 are each 0.33 μf to improve the rollof the filter.

Amplifier U2-D 554 is part of an auto-zero circuit to compensate fortemperature drift for the differential amplifier 302. A positive inputof the amplifier U2-D 554 connects to a reference voltage signal MID V,which is approximately ½ of the battery voltage, and is formed by aresistor divider coupled between the positive terminal and the negativeterminal and comprises resistors R16 558-R21 586.

A negative input of the amplifier U2-D 554 connects to the second end ofresistor R13 550 and a first pin of capacitor C1 556. An output of theamplifier U2-D 554 connects to a second end of resistor R12 520, asecond pin of capacitor C1 556, the second pin of capacitor C23 632, andthe second end of resistor R11 552.

In an embodiment, test points are provided. A first optional test pointat pin 1 of plug 7 on the PCB 510 monitors the signal COMP IN andconnects to a first end of resistor R30 548 and a second end of resistorR30 548 connects to the output of amplifier U2-C 540. A second optionaltest point at pin 2 of plug 7 monitors the auto zeroing circuit, whichprovides compensation for temperature drift. Pin 2 of plug 7 connects toa second end of resistor R31 562 and a first end of resistor R31 562connects to the output of amplifier U2-D 554.

Referring to FIG. 4 Block 402 and FIG. 5B, amplifiers U2-A and U2-Breceive the sensor signal. Referring to FIG. 4 block 404 and FIG. 5B,amplifiers U2-A, U2-B, and U2-C and their associated circuit componentsamplify the received sensor signal and output the amplified sensorsignal to comparators U1-A 546, U1-B 544.

Referring to FIGS. 3 and 5C, the comparator 306 comprises comparatorsU1-A 546, U1-B 544, and the voltage divider comprising resistors R16558-R21 586 and sensitivity jumpers JP1 572, JP2 564. The shut-offcircuit 308 comprises transistor Q1 612 and relay K1 584. ComparatorU1-A 546 pin 6 and comparator U1-B 544 pin 11 connect to the negativeand positive power supply voltages respectively and comparator U1-A 546pin 8 and comparator U1-B 546 pin 3 connect to the negative power supplyvoltage to provide power to comparators U1-A 546 and U1-B 544, as isknown to one of skill in the art.

The output of amplifier U2-C 540 (COMP IN) connects to a negative inputof the comparator U1-B 544 and a positive input of the comparator U1-A546. A first sensitivity jumper JP1 572 in series with resistor R22 566couples a positive input of comparator U1-B 544 to a negative input ofcomparator U1-A 546. In addition, a second sensitivity jumper JP2 564 inseries with resistor R23 570 couples the positive input of comparatorU1-B 544 to the negative input of comparator U1-A 546.

The positive input of comparator U1-B 544 further connects to a firstend of resistor R16 558, and a first end of resistor R19 568. Thenegative input of comparator U1-A 546 further connects to a first end ofresistor R17 560, and a first end of resistor R18 574.

Resistors R21 586, R19 568, R16 558, R17 560, R18 574, and R20 582connect in series and couple between a second end of R14 594 and asecond end of R15 580. The second ends of resistor R16 558 and resistorR17 560 connect and form the MID V signal, which is approximatelyone-half of the 12 volts, or approximately 6 volts. Capacitor C2 596connects across the resistor divider to provide decoupling. An anode ofzener diode D4 584 connects to the junction of resistors R18 574 and R20582, while a cathode of zener diode D4 584 connects to the junction ofresistors R21 586 and R19 568.

The value of the threshold is determined, at least in part, by thevalues of resistors R16-R23. Further, in an embodiment, sensitivityjumpers JP1 572 and JP2 564 can be optionally opened or closed to changethe sensitivity of the signal processor 112. Sensitivity jumpers JP1 572and JP2 564 adjust the sensitivity of the comparators U1-B 544, U1-A 546by adjusting the threshold at which the comparators U1-B 544, U1-A 546trip. This affects the distance between the sensor array 110 and thesafety device 114 that causes at least one operation of the machine 102to terminate.

In an embodiment illustrated in FIG. 5C, for comparative purposes, whenboth jumpers JP1 572, JP2 564 are closed, the system 100 is moresensitive than when jumper JP2 564 is closed and jumper JP1 572 is open.When jumper JP2 564 is closed and jumper JP1 572 is open, the system 100is more sensitive than when jumper JP2 564 is open and jumper JP1 572 isclosed. Finally, when jumper JP2 564 is open and jumper JP1 572 isclosed, the system 100 is more sensitive than when both jumpers JP1 572,JP2 564 are open.

When the threshold 304 is lower or in other words, when the controlcircuitry 112 is more sensitive, the safety device 114 is farther fromthe sensor array 110 in order to cause the machine 102 to shut-off thanwhen the threshold 304 is higher or the control circuitry is lesssensitive. When the threshold 304 is higher, or in other words, when thecontrol circuitry 112 is less sensitive, the safety device 114 is closerto the sensor array 114 in order to cause the machine 102 to shut-offthan when the threshold is lower or the control circuitry is moresensitive.

In another embodiment, by adjusting the resistor values R16-R23, thesensitivity of the signal processor 112 can be adjusted. By selectivelyopening or closing the jumpers JP1, JP2, the sensitivity of the signalprocessor 112 can be finely tuned to the desired response.

An output of the first comparator U1-B 544 connects to an output of thesecond comparator U1-A, a first end of resistor R24 606, and a cathodeof diode D1 608. A second end of resistor R24 606 connects to a cathodeof an indicator diode LED1 598 and an anode of the indicator diode LED1598 connects to first end of R14 594. In an embodiment, the indicatordiode LED1 598 is a blue light emitting diode (LED), which, when turnedon, indicates that the safety system 100 is tripped and the machine 102is off. In other embodiments, other colors of LEDs can be used. In yetother embodiments, other indicia of activation, such as a beeper, andthe like can be used. In further embodiments, there is no tripindicator.

An anode of diode D1 608 connects to a first end of resistor R27 610 anda base of transistor Q1 612. A second end of resistor R27 610 connectsto a first end of resistor R25 600, and a cathode of zener diode D5 628.A second end of resistor R25 600 connects to the first end of R14 594and an anode of zener diode D5 628 connects to the first end of R15 580.

An emitter of transistor Q1 612 connects to a first terminal of the coilof relay K1 584 and a second terminal of the coil connects to the firstend of R15 580.

A first terminal of a first contact of the relay K1 584 couples to thecutting machine 102 through pin 1 of plug 4 on the PCB 510 and throughterminal 5 of the terminal block 508, which carries a CONTROL 1 signal626 to the cutting machine 102. A second terminal of the first contactof the relay K1 584 couples to the cutting machine 102 through pin 2 ofplug 4 on the PCB 510 and through terminal 6 of the terminal block 508,which carries a CONTROL 2 signal 624 to the cutting machine 102.

A first terminal of the second pole of the relay K1 584 connects to ananode of an indicator diode LED2 620 and to a cathode of diode D6 618. Acathode of the indicator diode LED2 620 connects to a first end ofresistor R26 622 and a second end of the resistor R26 622 connects to acollector of the transistor Q1 612. A second terminal of the second poleof the relay K1 584 connects to the first end of R14 594.

In one embodiment, the relay K1 584 is a 2 pole, single throw, normallyopen and momentary action relay, such as relay part number CR-3402-5-71.In other embodiments, other relays with different features, such asdifferent numbers of pole and throws, normally closed, locking action,and the like can be used.

In an embodiment, indicator diode LED2 620 is a red LED. In otherembodiments, other colors of LEDs can be used. In yet other embodiments,other indicia, such as a beeper, strobe lights, and the like can beused.

Referring to FIG. 4 Block 406 and 5C, when the amplified sensor signalis greater than the threshold, the relay K1 584 deactivates causing thepoles to open, which disconnects the signals CONTROL 1 626 and CONTROL 2624 from each other. In addition, indicator diode LED 2 turns off toindicate that the safety system 100 has terminated an operation of themachine 102.

Referring to FIG. 4 Block 408, the cutting machine 102 receives thesignals CONTROL 1 626 and CONTROL 2 624 and shuts down. In anembodiment, the feed mechanism, the cutting blades, or both cease tooperate. In an embodiment, the relay K1 584 provides the power to theinfeed mechanism.

The use of the control signals CONTROL 1 626 and CONTROL 2 624 to shutoff the machine 102 depends on the configuration of the specificmachine. In other embodiments, there are other ways to use the CONTROL 1and CONTROL 2 signals 626, 624 to terminate an operation of the machine102, as is known to a person having ordinary skill in the art.

A first terminal of RESET switch 614 connects to an anode of diode D6618 through terminal 7 of terminal block 508 and pin 1 of plug 3 on PCB510. A second terminal of the RESET switch 614 connects to a first endof resistor R32 604 and a first pin of capacitor C20 602 throughterminal 8 of terminal block 508 and pin 2 of plug 3 on PCB 510. Asecond end of resistor R32 604 and a second pin of capacitor C20 602connect to the first end of R14 594.

Activating the RESET switch 614 resets the signal processor 112. Thecoil of relay K1 584 energizes and indicator diode LED 2 illuminatesindicating that the safety system 100 is again active. In an embodiment,the RESET switch 614 is a pushbutton switch, or the like

Table 1 indicates exemplary values the components of FIGS. 5A-5C,according to an embodiment. Unless otherwise indicated, resistors are ¼W.

R1 1 KΩ R2 1 KΩ R3 470 Ω R4 470 Ω R5 470 Ω R6 150 KΩ R7 150 KΩ R8 10 KΩR9 10 KΩ R10 100 KΩ R11 100 KΩ R12 3.3 KΩ R13 100 KΩ R14 10 Ω R15 10 ΩR16 100 Ω R17 100 Ω R18 1 KΩ R19 1 KΩ R20 100 Ω ½ W R21 100 Ω ½ W R22330 Ω R23 220 Ω R24 1 KΩ ½ W R25 330 Ω ½ W R26 220 Ω R27 4.7 KΩ R30 1.5KΩ R31 1.5 KΩ R32 10 KΩ R34 Selectable for offset bias R35 Selectablefor offset bias Q1 NTE123AP U1 LM319F U2 AD713JN F1 500 ma C1 1 μf C25.6 μf C3 0.1 μf C4 0.1 μf C5 0.22 μf C6 0.22 μf C20 2.2 μf C23Selectable for best response C24 Selectable for best response D1 1N914BD2 1N4001 D3 1N4001 D4 1N5232B D5 1N5232B D6 1N4001 LED1 Blue 276-006LED2 Red 276-086 K1 CR-3402-5-71

FIG. 6 illustrates an exemplary glove-type safety device 650. In anembodiment, the safety device 650 comprises a glove 652, such as a workglove, a magnet 654, and an indicator 656.

In some instances, chipping machine operators 108 may not wear gloves,as gloves have a possibility of catching on the branches and othermaterials being fed into the infeed chute 104. FIG. 7 illustrates anexemplary wrist-band/ankle bracelet safety device 700 that can be wornon the operator's limbs, such as around the wrist, ankle, or the like.FIG. 8 illustrates the safety device 700 worn on each shoe of a pair ofshoes, such that each safety device 700 is around each of the operator'sankles. In other embodiments, the safety device 650, 700 comprises anarticle, a member, an apparatus, or an item that permits the magnet 654to be removably attached to an operator's limb. In other embodiments,the safety device 650, 700 comprises an article, a member, an apparatus,or an item that is used to fasten the magnet 654 to a foreign body whichcould cause damage to the machinery if it was introduced into theworking parts of the machine 102.

The safety device 700 comprises a band 702, a fastening mechanism 704,at least one magnet 654, and the indicator 656. The safety device 700can comprise more or less than two magnets 654.

The magnet 654 can be any material or object that produces a magnetfield, such as permanent magnets, magnetizable materials,electromagnets, pulsating electromagnets, ferromagnetic materials,ferrimagnetic materials, and the like. Examples of permanent magnetmaterials are alnico, ferrite, neodymium, and the like and examples ofmagnetizable materials are iron, nickel, cobalt, lodestone, and thelike.

The magnet 654 can be any size or shape, such as a bar magnet, acircular magnet, a ring magnet, a cylindrical magnet, and the like. Amagnet typically has a north pole and a south pole. In general, thefarther apart the poles are on the magnet 654, the greater the inducedcurrent/induced voltage. Thus, when the poles on the magnet 654 arefarther apart, the safety system 100 has greater sensitivity.

The surface field of the magnet 654 can range between approximately4,000 gauss and approximately 10,000 gauss, and more preferably aroundapproximately 6,000 gauss.

In an embodiment, the magnet 654 is a ½″×¼″×2″ neodymium, grade N42, barmagnet.

For the better sensitivity, the magnets 654 located on the safetydevices 650, 700 have the same alignment. In an embodiment where thesafety device 650, 700 comprises more than one magnet 654, each magnet654 on the safety device 650, 700 has the same alignment. In anotherembodiment where the operator 108 is wearing more than one safety device650, 700, the magnets 654 in all of the worn safety devices 650, 700have the same alignment. In other words, the same pole of the magnet 654should be passing the sensor elements 110 a, 110 b regardless of whetherthe safety device 650, 700 is worn on the operator's hand, wrist, arm orfoot, ankle, leg. For example, as illustrated in FIGS. 6 and 8, when theoperator is wearing safety device 650 on his hand and safety device 700on each of his shoes, the same pole of the magnet 654 on each safetydevice 650, 700, which is the south pole in this example, will passthrough the sensor array 110 first when the operator 108 moves his handor either foot into the infeed chute 104.

In another embodiment, the north pole of the magnet 654 is positioned onboth of the safety devices 650, 700 closer to the operator's fingers andtoes. In other embodiments, the poles of the magnets 654 on the safetydevices 650, 700 are not aligned.

The indicator 656 is located on the safety device 650, 700 to indicatethat the machine operator 108 is wearing the safety device 650, 700. Inan embodiment, the indicator 656 is a band or stripe around the wristportion of the glove 652. In another embodiment, the indicator 656 is astripe along an edge of the band 702. In other embodiments, theindicator 656 is any material or emblem attached to the safety device650, 700 that provides a visual indication that the operator 108 iswearing the safety device 650, 700. Further, the indicator 656 indicatesthat the operator is wearing the safety device 700 in the correctorientation for maximum effectiveness. For example, if the bandindicator 656 of the safety device 700 worn on the operator's shoe is onthe top edge and if the band indicator 656 of another safety device 700worn on the operator's other shoe is on the bottom edge, then themagnets 654 in the two safety devices 700 are not aligned. This reducesthe sensitivity of the safety system 100.

In an embodiment, the indicator 656 is brightly colored, such as neongreen, fluorescent orange, bright blue, or the like, to allow easyidentification of the safety device 650, 700 from a distance.

In an embodiment, the band 702 comprises fabric. Other examples ofmaterials that can be used for the band 702 are plastic, paper, leather,or the like. When the operator 108 is wearing the safety device 700, thefastening mechanism 704 connects a first end of the band 702 to a secondend of the band 702 such that the safety device 700, when worn,surrounds the operator's limb. Examples of the fastening mechanism 704are hook and loop tape, straps, tape, ties, clips, and the like. Inanother embodiment, the band 704 is a continuous loop of elasticmaterial such that the operator slips the elastic safety device over hishand/foot and onto his wrist/ankle. In an embodiment, the fasteningmechanism 704 is configured to easily open in the event the safetydevice 114 becomes entangled in the branches being fed into the infeedchute 104.

Depending on the embodiment, certain acts, events, or functions of anyof the algorithms described herein can be performed in a differentsequence, can be added, merged, or left out all together (e.g., not alldescribed acts or events are necessary for the practice of thealgorithm). Moreover, in certain embodiments, acts or events can beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors or processor cores or onother parallel architectures, rather than sequentially.

The various illustrative logical blocks, modules, and algorithm stepsdescribed in connection with the embodiments disclosed herein can beimplemented as electronic hardware, computer software, or combinationsof both. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. The described functionality can be implemented invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the disclosure.

The various illustrative logical blocks and modules described inconnection with the embodiments disclosed herein can be implemented orperformed by a machine, such as a general purpose processor, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor can be a microprocessor,but in the alternative, the processor can be a controller,microcontroller, or state machine, combinations of the same, or thelike. A processor can also be implemented as a combination of computingdevices, e.g., a combination of a DSP and a microprocessor, a pluralityof microprocessors, one or more microprocessors in conjunction with aDSP core, or any other such configuration.

The steps of a method, process, or algorithm described in connectionwith the embodiments disclosed herein can be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. A software module can reside in RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, harddisk, a removable disk, a CD-ROM, or any other form of computer-readablestorage medium known in the art. An exemplary storage medium can becoupled to the processor such that the processor can read informationfrom, and write information to, the storage medium. In the alternative,the storage medium can be integral to the processor. The processor andthe storage medium can reside in an ASIC.

Conditional language used herein, such as, among others, “can,” “might,”“may,” “e.g.,” and the like, unless specifically stated otherwise, orotherwise understood within the context as used, is generally intendedto convey that certain embodiments include, while other embodiments donot include, certain features, elements and/or states. Thus, suchconditional language is not generally intended to imply that features,elements and/or states are in any way required for one or moreembodiments or that one or more embodiments necessarily include logicfor deciding whether these features, elements and/or states are includedor are to be performed in any particular embodiment. The terms“comprising,” “including,” “having,” and the like are synonymous and areused inclusively, in an open-ended fashion, and do not excludeadditional elements, features, acts, operations, and so forth. Also, theterm “or” is used in its inclusive sense (and not in its exclusivesense) so that when used, for example, to connect a list of elements,the term “or” means one, some, or all of the elements in the list.

While the above detailed description has shown, described, and pointedout novel features as applied to various embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the devices or algorithms illustrated can be madewithout departing from the spirit of the disclosure. As will berecognized, certain embodiments of the inventions described herein canbe embodied within a form that does not provide all of the features andbenefits set forth herein, as some features can be used or practicedseparately from others. The scope of certain inventions disclosed hereinis indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. A method of automatically interrupting the operation of a cuttingmachine, the method comprising: forming a sensor array including a firstsensor element and a second sensor element, the first sensor elementincluding a first winding, the first winding wound in a first direction,the second sensor element including a second winding, the second windingwound in a second direction, the first and second windings connected inseries opposing; mounting the sensor array on opposing sides of aninfeed chute of a cutting machine such that the first sensor element ismounted to a first side of the infeed chute and the second sensorelement is mounted to an opposing second side of the infeed chute;sensing a current induced in the windings by a magnetic field, themagnetic field produced by a safety device placed proximate to thesensor array; comparing a signal proportional to the induced current toa threshold; and terminating an operation of the cutting machine whenthe signal exceeds the threshold.
 2. The method of claim 1, whereinsensing a current induced in the windings comprises: inducing a firstcurrent in the first winding from the safety device positioned betweenthe windings; inducing a second current in the second winding from thesafety device positioned between the windings; adding the first andsecond induced currents; inducing a third current in the first windingfrom an external source; inducing a fourth current in the second windingfrom the external source; and subtracting the third current and thefourth current.
 3. The method of claim 1, wherein the first and seconddirections are clockwise and counterclockwise.
 4. The method of claim 1,wherein the windings each include a first end and a second end and thefirst ends of the windings are connected together.
 5. The method ofclaim 1, wherein the first winding and the second winding each comprisebetween approximately 12 and approximately 16 turns.
 6. The method ofclaim 1, wherein the safety device is worn by an operator on at leastone of the operator's limbs.
 7. The method of claim 1 wherein the safetydevice comprises at least one magnet.
 8. The method of claim 1, whereinthe signal is proportion to a sum of the current induced in the firstwinding and the current induced in the second winding.
 9. A safetysystem for a cutting machine comprising: a sensor array including atleast two sensor elements, each sensor element comprising a protectiveplate and a winding, the winding comprising at least one loop of wire,the sensor elements mounted on opposing sides of an infeed chute of acutting machine and connected in series opposing; and a signal processorcomprising: an amplifier to amplify a signal proportional to a currentinduced in the windings, the current induced by a magnetic field movingproximate to the sensor array; and a comparator to compare the amplifiedsignal to a threshold value, such that the signal processor causes thecutting machine to shut off power to an operation of the cutting machinewhen the amplified signal is greater than the threshold value.
 10. Thesafety system of claim 9, wherein the amplifier amplifies a signalproportional to a voltage potential induced in the windings.
 11. Thesafety system of claim 9, wherein at least one winding is woundclockwise and at least another winding is wound counterclockwise. 12.The safety system of claim 9, wherein a safety device moving proximateto the sensor array provides the magnetic field.
 13. The safety systemof claim 12, wherein the safety device comprises at least one magnet.14. The safety system of claim 9, wherein the threshold value comprisesa positive set point and a negative set point, and the signal processorcauses the cutting machine to shut off power to the operation of thecutting machine when the amplified signal is greater than the positiveset point or less than the negative set point.
 15. An apparatus forautomatically interrupting operation of a cutting machine comprising: asensor array including at least a first array element and at least asecond array element, each array element including a winding, the arrayelements connected in series opposing; an amplifier for amplifying asignal proportional to a sum of a current induced by a magnetic field inthe at least first sensor array element and a current induced by themagnetic field in the at least second sensor array element; a thresholdcircuit for generating a predetermined threshold; a comparator forcomparing the amplified signal to the predetermined threshold; and ashut-off circuit to terminate an operation of a machine when theamplified signal is greater than the predetermined threshold.
 16. Theapparatus of claim 15 further comprising sensitivity jumpers to adjustthe sensitivity of the threshold circuit.
 17. The apparatus of claim 15,wherein the current is induced by the magnetic field moving between theat least first sensor array element and the at least second sensor arrayelement.
 18. The apparatus of claim 15, wherein the at least first arrayelement is wound in a different direction than the at least second arrayelement.
 19. The apparatus of claim 15, wherein the predeterminedthreshold comprises a positive predetermined threshold set point and anegative predetermined threshold set point, and the shut-off circuitterminates the operation of the machine when the amplified signal isgreater than the positive predetermined threshold set point or when theamplified signal is less than the negative predetermined threshold setpoint.
 20. The apparatus of claim 15, wherein a safety device placedproximate to the sensor array provides the magnetic field.